In the semiconductor industry, nitrogen trifluoride has a variety of applications, including as a fluorine source for plasma etch, a cleaning agent for CVD reactors, and a dry etchant for semiconductors. Besides, nitrogen trifluoride is useful as a rocket fuel. With the development of the semiconductor industry, the nitrogen trifluoride gas has been in great demand. The increasing demand for a gas without any environmental pollution increased the demand for the nitrogen triflurode gas by geometric progression. As a matter of course, nitrogen trifluoride gas for use in these applications is required to be of high purity.
Nitrogen trifluoride can be prepared by various methods, including the direct fluoridation of ammonia, the use of plasma to react ammonia with fluorine, and molten salt electrolysis for electrolyzing NH4.xHF (x: 1.8˜2.1) with ammonium fluoride or hydrogen fluoride used as a material.
In most cases when using these methods, impurities, such as nitrous oxide (N2O), carbon dioxide (CO2), carbon tetrafluoride (CF4), dinitrogen difluoride (N2F2), etc., are concurrently produced in large amounts, so that purification is necessary to obtain highly pure NF3 gas suitable for use in the semiconductor industry.
Particularly, CF4, produced as an impurity upon the production of NF3, is difficult to be removed from NF3 by general methods such as distillation, bulk adsorption, etc., because of the similarities therebetween in boiling point, molecular size, and heat of adsorption. On the other hand, carbon tetrafluoride, even if present in a trace amount, causes troubles in semiconductor etching because it forms solid residues such as carbon or silicon carbide.
U.S. Pat. No. 5,069,887, yielded to Takashi et al., discloses a method of refining nitrogen trifluoride gas using synthetic zeolite in removing carbon tetrafluoride. In the method, zeolite 5A containing 1 to 10% by weight of crystallization water is used as an adsorbent to give nitrogen trifluoride with a carbon tetrafluoride content of 10 ppm or less, which meets the requirements of the electronic industry. This method, however, is disadvantageous in that the water content of the synthetic zeolite must be adjusted to within a predetermined range and the adsorption temperature range is very narrow.
Another refining method using synthetic zeolite can be found in U.S. Pat. No. 5,069,690, yielded to Philip et al., which discloses the kinetic gas-solid chromatography. In this method, hydrothermally treated zeolite having a certain porosity is used as an adsorbent, and discrete pulses of a mixture of gases are passed through a bed of the porous adsorbent that kinetically adsorbs one gas more readily than the other gases, resulting in the selective separation of NF3 gas. However, the hydrothermal pre-treatment for controlling the porosity of zeolite is difficult to conduct. In addition, the available time period of the zeolite, that is, the saturation time period, is too short to apply the zeolite method in the commercialized mass purification of nitrogen trifluoride.